Process of metal plating plastics

ABSTRACT

PROCESS FOR INCREASING THE ADHESION OF METAL PLATING TO A SURFACE OF A POLYMERIC SUBSTRATE WHICH COMPRISES INCORPORATING IN SAID POLYMER A SUFFICIENT AMOUNT UP TO AND INCLUDING TWO PERCENT BY WEIGHT, OF A LOW MOLECULAR WEIGHT ORGANIC COMPOUND, SAID LOW MOLECULAR WEIGHT COMPOUND HAS A MINIMUM OXIDATION RATE OF AT LEAST ABOUT TWENTY TIMES THE OXIDATION RATE OF STEARIC ACID; THEREAFTER SUFFICIANTLY OXIDIZING A SURFACE OF THE POLYMERIC SUBSTRATE TO OXIDIZE SAID ORGANIC COMPOUND THEREAT; AND THEREAFTER METAL PLATING SAID OXIDIZED SURFACE OF SAID SUBSTRATE TO PRODUCE A METAL PLATED SUBSTRATE IN WHICH THE METAL PLATING THEREON HAS A MINIMUM PEEL STRENGTH OF 5. METAL PLATED PLASTIC ARTICLES PRODUCED BY THIS PROCESS ARE DESCRIBED.

United States Patent I 3,556,955 PROCESS OF METAL PLATING PLASTICS Fred H. Ancker, Bound Brook, and Frederick L. Baier,

South Plainfield, N.J., assignors to Union Carbide Corporation, a corporation of New York No Drawing. Filed Feb. 18, 1966, Ser. No. 528,389

Int. 'Cl. C23b 5/60 US. Cl. 204-30 13 Claims ABSTRACT OF THE DISCLOSURE Process for increasing the adhesion of metal plating to a surface of a polymeric substrate which comprises incorporating in said polymer a sufficient amount up to and including two percent by weight, of a low molecular weight organic compound, said low molecular weight compound has a minimum oxidation rate of at least about twenty times the oxidation rate of stearic acid; thereafter sufiiciently oxidizing a surface of the polymeric substrate to oxidize said organic compound thereat; and thereafter metal plating said oxidized surface of said substrate to produce a metal plated substrate in which the metal plating thereon has a minimum peel strength of 5. Metal plated plastic articles produced by this process are described.

This invention relates to metal-plated plastics and to a process for the preparation thereof. More particularly, this invention relates to metal-plated plastics exhibiting high levels of adhesion and to a relatively rapid plating process therefor.

Metallic coatings on plastics have been used for many years for electrical and decorative purposes as, for example, in printed circuit boards and ornamental decorations. Recently, there has been increasing commercial interest in functional .uses, i.e., applications wherein metal coated plastic parts can replace current all-metal parts. In addition to being more economical to manufacture, metal coated plastic parts are superior to all-metal parts in many important respects. For example, chromium-plated plastics are more weather-resistant than chromium-plated metals since the plastic substrate is inherently non-corrosive; moveovenconsiderable weight can often be saved while still retaining a metallic appearance and feel.

i In order to realize these advantages, however, it is necessary to provide a high level of adhesion between the metal coating and the plastic substrate. If the requisite high level of adhesion is not obtained, the metal coating can blister or peel from the plastic substrate either when subjected to variations in temperature, due to the difference in thermal coeflicient of expansion between metal and plastic, or to small strains, due to the difference in elastic modulus. It has heretofore been empirically established in the plating art that a minimum peel strength of about 5 pounds per inch isrequired to prevent this type of failure in most applications of metal-plated plastic parts.

The methods currently employed for depositing metal coatings on plastic substrates usually involve the use of several treating and plating baths, Generally, the plastic substrate is first conditioned or treated in a strong oxidizing solution, e.g., chromic acid/sulfuric acid, and then sensitized in a solution of a reducingagent, e.g., stannous chloride. The substrate is thereafter activated by immersion in a dilute solution of a noble metal salt, e.g., palladium chloride, and then transferred to a so-called electroless plating bath wherein the substrate receives a sufiiciently conductive metal coating to permit subsequent electroplating by the conventional methods used for standard metal parts, Electroless plating solutions are Patented Jan.'1'9, 1971 meta-stable solutions of a metal salt, e.g., copper, nickel and the like, and a reducing agent, e.g., formaldehyde, hypophosphite, sodium borohydride, and the like, in which reduction of the metal ion is inhibited by complexing agents such as ammonia, hydrocarboxylic acids, and the like. The primary purposes of the pre-treatments are to improve the metal-to-polymer adhesion and to assure that the deposition of metal from the electroless plating solution occurs on the surface of the plastic substrate in preference to the walls of the plating bath.

The basic problem, however, which arises when metal deposition techniques such as described hereinabove are.

applied to currently available plastic substrates is that low adhesion values are generally obtained. Typical metal to polymer adhesions are shown in Table I below.

TABLE I Polymer: Peel strength (lbs/in.) Acetal 0.5 Acrylic 1.5 Polystyrene (high impact) 0.8 ABSstandard grades 0.8-2.5

ABS-plating grade 5.0-10.0

These and other objects are accomplished by the present invention which provides metal-plated plastic substrates exhibiting a minimum peel strength of at least about 5.0 pounds per inch. These metal-plated plastic substrates are produced by a process for increasing the metal-to,- polymer adhesion which comprises incorporating into the polymer up to and including about two percent by weight of a low molecular weight organic compound having a minimum oxidation rate of at least twenty times, and

more preferably at least fifty times the oxidation rate of stearic acid; oxidizing the resulting plastic substrate and thereafter metal plating said oxidized plastic substrate.

The polymeric materials which can be effectively plated by the process of the present invention are thermoplastics such as the polyolefins, for example, polyethylene, polypropylene, poly(butene-l), poly(vinyl chloride), poly- V (vinylacetate), polystyrene, polyacrylonitrile, polyfornial,

poly(acrylic and methacrylic acid esters) such as polyethyl acrylate) and poly(methyl methacrylate), and co- I polymers thereof.

It has been found in the present invention, that certain classes of low molecular weight organic compoundshereinafter referred to as latent adhesion promoterswhen employed in concentrations of about two percent or even less of the total polymer, provide adhesion values with metal platings which are substantially higher than,

any heretofore attainable for metal plated plastics. These latent adhesion promoters are characterized by having an moter into at least the surface of the vplastic article. Thus,

incorporated as used herein is intended to encompass compounding or blending the adhesion promoter into the polymer prior to forming the polymer into a desired article as well as impregnating at least the surface of a pre-formed polymeric article. Since the adhesion is latent until an oxidation treatment, the adhesion promoters can be employed in resin compositions without creating processing difficulties due to excessive sticking in the equipment, in contrast to many current adhesive and coating resin compositions. Actually, several of the preferred adhesion promoters behave as lubricants during processing, facilitating release of the hot plastic from steel equipment and molds. It has even been possible to incorporate a sufiicient amount of the adhesion promoters into the surface of molded plastics by using them as mold release sprays.

The .ability of an organic compound to promote adhesion has been found to correlate closely with its susceptibility toward autoxidation, i.e., with the reactivity of one or more of its hydrogen atoms towards hydroperoxide formation. As is well known, the chemical reactivity of a hydrogen atom in a carbon-hydrogen bond is strongly influenced by alkyl and aryl substituents on the same carbon atom. Similarly, adjacent double bonds, especially in alicyclic ring structures, markedly increase the hydrogen reactivity as does adjacent oxygen atoms, e.g., ether linkages. While these generalizations are helpful, they are not without exception and fail to correlate accurately with the oxidation rates of more complicated molecules. Accordingly, it is not considered satisfactory nor feasible to define the latent adhesion promoters of the present invention in terms of chemical structure alone. Instead, it is more meaningful and more readily susceptible for convenient determination to define the requisite reactivity in relation to the oxidation rate of a well-defined chemical compound taken as a standard, namely, stearic acid. Thus, the latent adhesion promoters of the present invention are low molecular weight organic compounds having a minimum oxidation rate at least about twenty times greater than the oxidation rate of stearic acid and preferably at least about fifty times greater than the oxidation rate of stearic acid. The relative oxidation rates can be readily determined by methods such as that described by A. I. Stirton et al. in Oil and Soap, 22, pages 81-83 (1945). The molecular weight of the adhesion promoters can range up to about 2000 and preferably is between 100 and 2000.

Examples of typical latent adhesion promoters are low molecular weight organic compounds such as the highly unsaturated fatty acids, e.g., sorbic acid, linoleic acid, linolenic acid, elaeostearic acid and liconic acid and their esters, amides and imides together with the amides and imides of mono-unsaturated fatty acids such as oleic and ricinoleic acid; highly unsaturated aliphatic hydrocarbons such as squalene; highly unsaturated alicyclic compounds such as abietic acid; aliphatic polyethers such as polyethylene glycol, polypropylene glycol as well as their adducts and esters, as for example the poly(ethylene oxide) adducts of nonyl phenol; tertiary aliphatic compounds such as isobutyric and isovaleric acid and the esters, amides and imides thereof and aliphatic substituted aromatic compounds containing at least one benzylic hydrogen such as cumene, thymol and their derivatives. It has also been found possible to employ such compounds in more or less natural or crude form, for example even by direct use of such products as linseed oil, tung oil, tall oil, wood rosin, and the like.

It is often desirable to tailormake the melting point, boiling point and compatibility of a latent adhesion promoter for a given polymer and/ or a given set of processing conditions. As is readily seen, suitable derivatives can readily be made from and within the above-described classes of compounds as long as the oxygen sensitive groups are retained. For example, reactive alcohols such as sorbic alcohol or polyethylene glycol can be used for as a result of oxidation in the presence of an adhesion.

esterification with reactive or non-reactive acids, polyalkylene oxide adducts can be made from compounds with suitable functional groups, etc.

Although not intending to be bound by any theory or mechanism, it is presently believed that the high levels of adhesion obtained by way of the present invention are attributable to the formation of carboxyl groups which are firmly attached to the polymer molecule. Freely available acidic groups on the plastic surface are believed to be a prerequisite to attaining high levels of adhesion to subsequently deposited metal coatings. Acidic groups have long been known to improve adhesion between polymers and metals. Accordingly, a variety of standard adhesive and coating resins have heretofore been prepared containing carboxyl groups in the polymer backbone, e.g., the maleic acid adduct of po1y(vinyl chloride), the maleic acid adduct of polystyrene, the maleic acid adduct of polyethylene, acrylic acid copolymers of ethylene, and the like. Quite unexpectedly, however, none of the above resins provide good adhesion to deposited metal coatings. This anomaly illustrates what is considered to be an im-. portant distinction between depositing a polymer on a metal surface, as in adhesion and coatings applications, and the reverse process of depositing a metal coating on a polymeric surface as encompassed by the present invention. When a polymer is applied to a metal surface in the molten or dissolved state, the carboxyl groups have a high degree of mobility and therefore are readily 'able: to orient themselves towards the metal. In a solid molded part, however, the carboxyl groups have a low mobility and, due to random orientation, are mainly imbedded within the surface. Carboxyl groups generated by surface reactions, however, are freely available and therefore more effective in developing adhesion to subsequently applie metal coatings.

The level of surface treatment required to attain high metal adhesion to a plastic substrate is substantially higher than that required to attain water wettability and ink adhesion. Thus, even treatments with aggressive chemicals for prolonged periods have been found inadequate with many polymers. For example, treatment of solid, isotactic polypropylene in a concentrated chromium trioxide/sulfuric acid solution failed to produce a satisfactory surface even after long immersion times. However, when oxidized with an adhesion promoter in accordance with the teach ings of the present invention, a dramatic increase in metal to polymer adhesion is obtained as shown hereinbelow.

The carboxylation of the plastic surface which occurs promoter can be followed by surface reflection infra-red techniques. Without wishing to be bound by any theory.

or mechanism, it is presently believed, that the adhesion.

promoter is preferentially oxidized into carboxyl containing free radicals which graft onto the less easily oxidizable polymer by an oxidative coupling mechanism. Since the adhesion promoter is readily oxidized into carboxyl conoxidation of the polymer surface is believed to occur,

catalyzed by the free radicals formed from thermal. cleavage of hydroperoxides formed during the early stages of oxidation of the adhesion promoter.

Most if not all of the preferred adhesion promoters disclosed in this invention have only limited compatibility with high polymers at room temperature and are as such subject to syneresis or exudation. Interestingly enough,

such compounds have heretofore been considered quite detrimental to metal-to-polymer.adhesion, for instance see I. J. Bikerman, J. Appl. Chem, 11, 81-85 (1961).

It has been found in the present invention that impreg-y nation or compounding of an adhesion promoter per se.

into a polymer without any special precautions usually does not improve adhesion because of adhesive failure between the exudate and the polymer. When, however, the excess exudate is removed from the polymer surface either by careful washing with a detergent or preferably with a solvent which is a good solvent for the adhesion promoter and a poor solvent for the polymer, then extremely high adhesions are possible on a completely reliable and reproducible basis. Also, in no case was a subsequent decrease in adhesive strength observed either after extensive thermal cycling or storage of the metal plated samples for more than a year.

Since the latent adhesion promoters are all compounds which are very readily subject to auto-oxidation, it is often desirable and indeed sometimes essential to prevent premature oxidation by addition of suitable antioxidants. This is done whether the adhesion promoters are used as compounding ingredients or for impregnation of molded parts, and is illustrated in some of the following examples.

Oxidation of the resin compositions of the present invention can be accomplished through use of oxidizing techniques which readily oxidize the latent adhesion promoter to carboxyl containing radicals. Oxidizing solutions found suitable for use in the present invention are aqueous solutions of chromic acid in inorganic acids or aqueous solutions of chromic acid, both being at least about 85% saturated with respect to chromic acid at the particular use temperature of the oxidizing bath. For example, an oxidizing bath containing 29 parts chromium trioxide, 29 parts concentrated sulfuric acid and 42 parts of water has been found useful. Other oxidizing solutions and techniques, however, can similarly be employed such as flame treatment, corona discharge, glow discharge, ozonation, or exposure to actinic or high energy radiation and the like in a manner and duration sufiicient to oxidize the latent adhesion promoter to carboxyl-containing radicals.

A conductive metal coating can be deposited on the oxidized polymer thereby permitting subsequent conven tional electroplating techniques to be employed to obtain an electroplated polymeric substrate exhibiting minimum peel strengths of at least about 5.0 pounds per inch. The conductive metal coating can be deposited by immersing the oxidized polymer in a solution of a reducing agent such as stannous chloride to sensitize said polymer. The-sensitized polymer can then be immersed in a solution containing a salt of a noble metal such as platinum, palladium, silver and gold and preferably in the form of a halide such as palladium chloride to activate said polymer. In lieu of the sensitizing and activatingbaths, other means can be employed to deposit an initial metal film in preparation for subsequent electroless metal depos'ition. For example, such films can-be deposited byspray gun systems, gas plating, cathode sputtering, vacuum metallizing, decomposition of metal carbonyls and the like. Thereafter, the polymer can be immersed in an electrolessplating solution such as those composed of a copper salt, a complexing agent'to keep the'coppe'r in solution and a reducing agent to deposit a conductive metal film on said polymer. Finally, the resulting polymeric substrate having a conductive metallic film thereon can be electroplated by conventional techniques which EXAMPLES 1-3 The following examples illustrate the effectiveness of highly unsaturated fatty acid esters as latent adhesion promoters when incorporated into polypropylene.

. polypropylene.

2500 grams of polypropylene having a specific gravity of 0.905 (ASTM D792-50) and a'melt-fiow (230 (32/ 44 p.s.i.) of 4 decigrams per minute (ASTM D1238) were heated and sheared in an eight pound steam heated Banbury mixer. After the resin was well fiuxed, 25 grams of castor oil (88% glycerol ester of ricinoleic acid) were added as a latent adhesion promoter in small portions to the molten resin and after the last addition, mixing was continued for two minutes. The resin compound was then transferred to a two roll mill maintained at 165 170 C., sheeted off, cooled and granulated. The resin granules were compression molded in an hydraulic press by preheating to 190 C. for 5 minutes under low pressure, then increasing the pressure to 500 p.s.i. and cooling to room temperature by water cooling the press platens.

The resulting 125 mil plaque was cleaned in a slightly alkaline aqueous soap solution, oxidized by immersion in a saturated aqueous chromic acid solution maintained at C. for 5 minutes to potentiate the adhesion promoter and then washed in deionized water to remove excess chromic acid. The plaque was next immersed in an acidified stannous chloride solution (Enthone sensitizer 432) at room temperature for one minute to sensitize the plaque and again washed in deionized water. Thereafter the plaque was immersed in a dilute solution of palladium chloride (Enthone activator 480) for one minute to activate the plaque, again washed in deionizedwater and then immersed in an electroless copper plating solution (Enthone Enplate Cu 400 A+B) at room temperature for ten minutes to obtain a conductive copper deposit. Finally, about two mils of ductile copper were deposited by electroplating using a bright acid copper bath (Udylite Ubac #1), a bath temperature of 27 C. and a current density of 60 amps per square foot.

The above procedure was repeated employing linseed oil and tung oil as the latent adhesion promoters. A-

control plaque of polypropylene containing no latent adhesion promoter was also subjected to the same procedure. The resulting metal-to-plastic peel strengths of the respective plaques were subsequently determined by scoring a one inch wide strip in the plaque, lifting the end of the metallic strip, clamping a weight pan to the end of the strip, and adding weights to apply a tensile load at an angle of to that of the metal-plastic interface until the metal strip peeled from the plastic substrate. The peel strengths thus obtained are shown in Table II:

The above table illustrates thecorrelation between adhesion and aliphatic unsaturation within the fatty acid series. Little or no effect was found with substantially saturated aliphatic fatty acid derivatives and hydrocarbons.

EXAMPLES 4-s The following examples illustrate the effect of low molecular weight aliphatic polyethers (liquids and/or waxes) when employed as latent adhesion promoters in Employing the same methods described in Examples 1-3, 1% of the-polyethylene glycol derivatives defined in Table III below were incorporated into polypropylene. The resulting peel strengths were obtained above. I t

as described 1 Pollyggiylene glycol having a molecular weight of about 1500 (Carbowax 2 Polyethylene oxide adduct of nonyl phenol (Tergitol" N 1 -44).

Improvement in adhesion was found generally to decrease as the molecule weight of the polyether additives increased unless the effective molecular weight was reduced due to prolonged heating of the compound.

. EXAMPLES 6-8 The following examples illustrate the efiect of highly unsaturated fatty acid esters when incorporated into polypropylene by impregnation.

A compression molded polypropylene plaque having a thickness of 125 mils was prepared from the polypropylene resin described in Example 1 but containing no latent adhesion promoter. The plaque was cleaned in a slightly alkaline, aqueous solution and then immersed for 3 minutes in a bath containing one of the unsaturated fatty acid esters described in Table IV below to impregnate the plaque therewith. The bath was maintained at 135 C. The impregnated plaque was cooled to room temperature and excess ester was washed 01f in a slightly alkaline soap solution. The plaque was then washed in deionized water and subsequently immersed for 5 minutes in a chromic acid/sulfuric acid oxidizing bath containing 29 parts of chromium trioxide, 29 parts of concentrated sulfuric acid and 42 parts of deionized water to oxidize the plaque and potentiate the adhesion promoter. The bath was maintained at a temperature of 80 C. Residual oxidizing solution was removed by washing the plaque with deionized water. Thereafter, the plaque was sensitized with staunous chloride solution, activated with palladium chloride solution, immersed in an electroless copper solution to obtain a conductive copper deposit and finally electroplated as described in Example 1.

Table IV below illustrates the adhesion values obtained by impregnation with the latent adhesion promoters described. The control plaque was treated in an identical manner to the other examples except for omission of the impregnation step.

TABLE IV Latent adhesion promoter Peel impregnation strength, Example bath lbs/in.

Control. None..- O 6 Castor oil 12 7 Linseed oil- 23 8 Tung oil 31 EXAMPLE 9 Although the foregoing examples have illustrated the use of sensitizing and activating solutions to deposit an initial metal film on-the plastic plaques, it is readily apparent that other means of depositing such films can similarly be employed and .high levels of adhesion still obtained. Thus, such films can be deposited by spray gun systems,

gas plating, cathode sputtering, vacuum metallizing, de composition of metal carbonyls and the like.

This example illustrates the efiect of a latent adhesion promoter when the initial metal layer is deposited by physical means, i.e., cathode sputtering, rather than chemi-' cal means 'as in the previous examples.

An injection molded polypropylene plaque (A) was impregnated in linseed oil, oxidized in a chromic acid/ sulfuric acid bath and washed as described in Examples 6-8. After drying, a very thin layer of palladium metal was deposited on the surface of the plaque by conventional cathode sputtering techniques. The sample was then immersed in an electroless copper bath and electroplated as described in Example 1. An identical polypropylene plaque (B) was treated exactly as (A) except'that the linseed oil impregnation step was omitted. The plaques were tested for metal-to-plastic adhesion, the results being shown in Table V below:

TABLE V Example: Peel strength (lbs./ in.) 9 (Plaque A) 18 Control (Plaque B) O EXAMPLES 10-13 The following examples illustrate the effect of unsaturated fatty acids as latent adhesion promoters when incorporated into polypropylene by compounding.

Compression molded polypropylene plaques were prepared in the same manner as described in Examples 1-3 containing one percent of the fatty acids shown in Table VI below. Excess adhesion promoter was carefully removed from the surface of the plaques by washing with diethylene glycol monoethyl ether acetate at room temperature followed by a water rinse. The plaques were then immersed for 15 minutes in the oxidation bath described in Examples 6-8. Residual oxidizing solution was removed by washing with deionized water. Thereafter, the plaques were electroplated by the sequence described in Example 1.

Table VI below illustrates the adhesion values obtained. The control plaque was treated in an identical manner to the other examples except it contained no adhesion promoter.

TAB LE VI Pe Latent adhesion strength promoter lbs./in

Example:

Oontr 10.-.. stearic acid- 11-- Oleic acid 4. 12 Linoleic acid 7. 11.

Tung oil fatty acid The above examples illustrate the necessity of employing a latent adhesion promoter having at least about 20' times the oxidation rate of stearic acid in order to obtain the minimum acceptable peel strength of 5 lbs./ in.

EXAMPLE 14-15 The following examples illustrate the use of fatty acid amides as latent adhesion promoters in polypropylene,

Employing the same procedures described in Examples 10-13, plaques were prepared and plated containing one :resulting peel strengths obtained are shown in Table VIL:

TABLE VII Peel strength Peel strength Peel strength with antioxiwith antioxi- (no antioxidant No. 1 dant N o. 2

Example Latent adhesion promoter dent) lbs/in. lbs/in. lbs./in.

C ontrol 14 Oleamide 8. 8 l4. 0 20 15 N-ethoxylated oleamide... 19. 4 24 Norn: Antioxidant #1 N,N'-diphenylpara-pheny1ene diamine. Antioxidant #2 tri(nonyla-tedpheuyDphosphite.

EXAMPLE 16-17 The following examples illustrate the use of cumene derivatives and unsaturated hydrocarbons as latent adhesion promoters in polypropylene.

Employing the same procedures described in Examples 10-13, plaques were prepared and plated containing one percent of p-isopropyl benzoic acid and squalene, respectively, compounded therein. In addition, 0.2'percent of N,N'-diphenylpara-phenylene diamine (antioxidant #1) was compounded therein to prevent premature oxidation of the latent adhesion promoters. The resulting peel, strengths obtained are shown in Table VIII.

TABLE VIII Peel strength Peel strength (no antioxi- (antioxidant Example Latent adhesion promoter dant) lbs/in. No. 1) lbs/m.

p-iso ropyl benzoic acid-.- 5. 8 12.0

Squa ene 5.4 24

EXAMPLE 18 This example illustrates that carboxylationoccurs at, the surface of the polymeric substrate as a result of 'the oxidation thereof in the presence of an adhesion promoter to produce freely available carboxyl groups firmly attached to the polymer molecule. The existence of freely available acidic groups at the surface of the substrate is believed to be a prerequisite to attaining high levels of adhesion.

Reflectance infrared spectra were obtained on a Perkin- Elmer Model 221 spectrophotometer using a Wilks Model 12 multiple reflectance attachment. In this technique, the sample was forced into physical contact with a crystal of high refractive index. The spectrum recorded was that of the surface of the sample, a few wavelengths in depth at 5 i.e. the effective thickness was about 15 1., or about 0.6 mils.

(A) A control sample of polypropylene was formed into a plaque as described in Example 1. The infrared refiectance scan of the sample showed the normal polypropylene bands, a weak carbonyl (5.7 1.), and a weak, broad adsorption at 6.06.3;i.

(B) A polypropylene plaque was impregnated with linseed oil as described in Example 7. The infrared reflectance scan of the impregnated sample showed a carbonyl absorption (5.7 1.) about ten times more intense than that of the control.

(C) A polypropylene plaque was impregnated with linseed oil and oxidized in the aqueous chromic acid/ sulfuric acid bath as described in Example 7. The infrared reflectance scan of the oxidized sample showed at least two different carbonyl absorptions, (5.7 and 5.8

(D) A polypropylene plaque was impregnated with linseed oil, oxidized in an aqueous chromic acid/sulfuric acid bath and copper plated as described in Example 7. Thereafter, the copper plate was stripped off with acid. The infrared reflectance scan showed at least two carbonyl bands (5.7 and 5.8a).

(E) A polypropylene plaque was treated as described in (D) followed by hydrolysis with potassium hydroxide. The infrared reflectance scan showed no carbonyl bands but a new absorption typical of a carboxylate group (6.3

(F) A polypropylene plaque was treated as described in (E) followed by reacidification with hydrochloric acid. The infrared reflectance scan indicated carbonyl absorptions at 5.7 and 5.8 No clear indication of carboxylate was observed.

(G) A polypropylene plaque was treated as described in (C) and then hydrolyzed with potassium hydroxide.-

The infrared reflectance scan showed no significant carbonyl band; however, carboxylate group absorption was observed at about 6.3

(H) A polypropylene plaque was treated as described in (G) and then acidified with hydrochloric acid. The

infrared reflectance scan showed at least two carbonyl hydrolysis and the reappearance of carboxyl groups upon reacidification.

EXAMPLE 19 Employing substantially similar procedures as described in Examples 1043, plaques of polystyreneare prepared and plated containing one percent of p-isopropyl benzoic acid. In addition, 0.1 percent of phenyl fl naphthyl amine is compounded therein to prevent premature oxidation of the latent adhesion promoter. The resulting peel strength is in excess of 5 pounds per inch.

EXAMPLE 20 Employing substantially similar prtmedures as described in Examples 10-13, plaques of vinyl chloride/vinyl acetate copolymer containing about vinyl chloride and 5% vinyl acetate are prepared and plated containing one percent abietic acid amide. In addition, 0.1 percent phenyl B naphthyl amine was compounded therein to prevent premature oxidation of the latent adhesion promoter. The resulting peel strength is in excess of 5 pounds per inch.

The metal plated plastics of the present invention are useful in the appliance fields as knobs and/ or handles for radios, televisions, refrigerators, housing, trim and/or components of refrigerators, vacuum cleaners, air conditioners, office machines; radio grills and cases and to replace zinc castings. They are also useful in the automotive fields as radio knobs and push buttons, instrument cluster housings, garnish moldings, window cranks and handles, and control levers. They are further useful in the hardware field as knobs, electrical outlet face-plates, toys, housings and the like. Also, they are useful for more demanding applications of printed electrical circuit boards, especially where direct soldering to metal panels are re quired.

What is claimed is:

1. Process for incfeasin'g the adhesion of metal plating to a surface of a polymeric substrate which comprises incorporating in said polymer a sufficient amount up to and including two percent by weight, of a low molecular weight organic compound which when the polymeric substrate is oxidized and metal plated, the metal plating has a minimum peel strength of 5 pound per inch, said low molecular weight compound has a minimum oxidation rate of at least about twenty times the oxidation rate of stearic acid; thereafter sufficiently oxidizing a surface of the polymeric substrate to oxidize said organic compound thereat whereby when said substrate is metal plated on said surface, said metal plating has a minimum peelstrength of and thereafter metal plating said oxidized surface of said substrate to produce a metal plated substrate in which the metal plating thereon has a minimum peel strength of 5.

2. Process as defined in claim 1 wherein said low molecular weight organic compound has a minimum oxidation rate of at least fifty times the oxidation rate of stearicacid.

3. Process as defined in claim 1 wherein said low molecular weight organic compound is incorporated into at least the surface of the polymeric substrate.

4. Process as defined in claim 3 wherein said low molecular weight organic compound is incorporated into at least the surface of the polymeric substrate by compounding it therein prior to forming said substrate.

5. Process as defined in claim 3 wherein said low mo lecular weight organic compound is incorporated into at least the surface of the polymeric substrate by impregnating the substrate therewith.

6. Process as defined in claim 1 wherein the molecular Weight of said low molecular weight organic compound ranges up to about 2000.

7. Process as defined in claim 6 wherein the molecular weight of said low molecular weight organic compound is between about 100 and about 2000.

8. Process as defined in claim 1 wherein said low molecular weight organic compound is a member selected from the group consisting of highly unsaturated fatty acids, the esters, amides and irnides thereof; mono-unsaturated fatty acid amides, aud imides, highly unsaturand imides thereof; and aliphatic substituted aromatic compounds containing at least one benzylic hydrogen.

9. Process as defined in claim 1 wherein an antioxidant is employed to prevent premature oxidation of said low molecular weight organic compound.

10. Process as defined in claim 1 wherein the resulting polymeric substrate is oxidized upon immersing said substrate in an oxidizing solution selected from the group consisting of aqueous solutions of chromic acid in inorganic acids and aqueous solutions of chromic acid, said solutions being at least about percent saturated with respect to chromic acid at the use temperature of the oxidizing'solution. Y

' 11. Process as defined in claim 1 wherein the polymeric substrate is polypropylene.

12. The process of claim 1 wherein after oxidizing the JOHN H. MACK, Primary Examiner W. B VANSISE, Assistant Examiner 

